Where I work we use Lorann Oils for flavoring candy. It's not the same as beer obviously, but I can tell you that yes, some of the flavors are more potent then others.

On the flavoring bottles they usually have printed how much you should use (as a percentage of the total batch size). I would recommend starting well below that amount and adding more to taste (since you only want a hint of flavor). Maybe even splitting your batch in case you add too much.

I've used 1 dram of watermellon to 5 gallons and I thought it was great. I also think watermellon is the only flavor I liked that I have tried from loranns. You may consider 1/2 dram in secondary then add the other half at bottling if you want more.

I have a few of these but never used them: watermelon, tangerine, pomegranate, pineapple and mango.
Will be making a cream ale this week. Was thinking 1/2 dram in half of a 5 gallon batch at bottling. SWMBO wants mango , but I prefer the watermelon.